Intake air of a gasoline engine flows through a filter of an air cleaner and then flows from an inlet of a carburetor into an intake passage. In the carburetor, a mixed air of atomized gasoline and the intake air is formed and flows into an engine body. Due to a change in a pressure in the engine body and the like, a phenomenon where the mixed air, including lubrication oil, flows through the intake passage in the reverse direction may occur. This phenomenon is called a “reverse flow”. In a two-stroke gasoline engine, when a pressure in a crankcase becomes high from moving a piston from top dead center to bottom dead center, most of the mixed air in the crankcase is supplied through a scavenging passage into a cylinder chamber, while a part of the mixed air may flow through the intake passage in the reverse direction together with the lubrication oil.
When the mixed air flowing through the intake passage in the reverse direction with the lubrication oil reaches the filter, the lubrication oil and the gasoline adhere to the filter. The gasoline adhering to the filter is evaporated and supplied again to the engine body together with intake air, while the lubrication oil is left on the filter, causing the filter to clog. Thus, as the operation period of the engine becomes long, clogging of the filter due to the lubrication oil may reduce the power output of the engine. Specifically, clogging of the filter increases flow resistance through the filter, reducing the amount of air so that a boost pressure of a fuel nozzle of the carburetor increases and thus the flow rate of the fuel increases. As a result, combustion in the engine is performed in a state of an excess fuel concentration such that the output of the engine is reduced. An operator has to replace the filter when the output of the engine is reduced due to the clogging of the filter causing an output performance to become worse.
In order to prevent the filter from being clogged by the lubrication oil, a technology of providing a reverse-flow restriction plate at the inlet of the carburetor has been known (for example, please refer to Patent Publications 1 and 2). Just after air-fuel mixture flowing through the intake passage in the reverse direction together with lubrication oil has passed through the inlet of the carburetor, it hits against the reverse-flow restriction plate and thus the amount of lubrication oil reaching the filter is reduced. A part of the lubrication oil adhering to the reverse-flow restriction plate is returned to the engine body together with intake air.